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From: Junio C Hamano <gitster@pobox.com>
To: Michael Haggerty <mhagger@alum.mit.edu>
Cc: Stefan Beller <sbeller@google.com>,
	git@vger.kernel.org, jamill@microsoft.com, mh@glandium.org
Subject: Re: [PATCH] xdiff: reduce indent heuristic overhead
Date: Tue, 03 Jul 2018 11:14:26 -0700	[thread overview]
Message-ID: <xmqq1sckrxtp.fsf@gitster-ct.c.googlers.com> (raw)
In-Reply-To: <72ac1ac2-f567-f241-41d6-d0f83072e0b3@alum.mit.edu> (Michael Haggerty's message of "Sun, 1 Jul 2018 17:57:35 +0200")

Michael Haggerty <mhagger@alum.mit.edu> writes:

> So if `N ≫ M`, there is necessarily a lot of repetition among the `N +
> M` lines that the hunk could possibly overlay. Specifically, it must
> consist of `floor((N + M)/M)` identical copies of the hunk, plus
> possibly a few leftover lines constituting the start of another repetition.
>
> Given this large amount of repetition, it seems to me that there is
> never a need to scan more than the bottom `M + 1` possible positions [1]
> plus the highest possible position [2] to be sure of finding the very
> best one. In the pathological case that you described above, where `M`
> is 1, only three positions have to be evaluated, not 100.

Nicely analysed.

  parent reply	other threads:[~2018-07-03 18:14 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-06-29  9:44 fast-import slowness when importing large files with small differences Mike Hommey
2018-06-29 20:14 ` Stefan Beller
2018-06-29 20:28   ` [PATCH] xdiff: reduce indent heuristic overhead Stefan Beller
2018-06-29 21:17     ` Junio C Hamano
2018-06-29 23:37       ` Stefan Beller
2018-06-30  1:11         ` Jun Wu
2018-07-01 15:57     ` Michael Haggerty
2018-07-02 17:27       ` Stefan Beller
2018-07-03  9:15         ` Michael Haggerty
2018-07-27 22:23           ` Stefan Beller
2018-07-03 18:14       ` Junio C Hamano [this message]
2018-06-29 20:39   ` fast-import slowness when importing large files with small differences Jeff King
2018-06-29 20:51     ` Stefan Beller
2018-06-29 22:10 ` Ævar Arnfjörð Bjarmason
2018-06-29 23:35   ` Mike Hommey
2018-07-03 16:05     ` Ævar Arnfjörð Bjarmason
2018-07-03 22:38       ` Mike Hommey

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